The present invention relates to a process for producing compact and preferably cellular polyurethane elastomers based on polyisocyanate mixtures containing 3,3xe2x80x2-dimethylbiphenyl 4,4xe2x80x2-diisocyanate, hereinafter also referred to in abbreviated form as PU elastomers, by reacting
a) relatively high molecular weight polyhydroxyl compounds and, if desired,
b) low molecular weight hydroxyl-containing chain extenders and/or crosslinkers with
c) 3,3xe2x80x2-dimethylbiphenyl 4,4xe2x80x2-diisocyanate and at least one additional aromatic diisocyanate selected from the group consisting of tolylene diisocyanate, diphenylmethane diisocyanate, 1,2-diphenylethane diisocyanate and phenylene diisocyanate, and/or at least one aliphatic diisocyanate having from 4 to 12 carbon atoms and/or at least one cycloaliphatic diisocyanate having from 6 to 18 carbon atoms, where the formative components (a), (c) and, if used, (b) are preferably reacted by the prepolymer method,
in the absence or preferably in the presence of
d) catalysts,
e) blowing agents and
f) additives,
and isocyanate prepolymers suitable for this purpose, preferably those based on diphenylmethane 4,4xe2x80x2-diisocyanate (MDI) and 3,3xe2x80x2-dimethylbiphenyl 4,4xe2x80x2-diisocyanate (TODI).
The microcellular PU elastomers have excellent static and dynamic properties. Owing to their specific damping characteristics and long-term use properties, they are used particularly in vibration- and shock-damping systems.
The production of compact or cellular, eg. microcellular, PU elastomers has been known for a long time from numerous patent and literature publications.
Their industrial importance is based on the combination of good mechanical properties with the advantages of low-cost processing methods. The use of various types of chemical formative components in different mixing ratios enables the production of thermoplastically processible or crosslinked, compact or cellular PU elastomers which have a wide variety of processing behaviors and mechanical properties. An overview of PU elastomers, their properties and uses is given, for example, in the Kunststoff-Handbuch, Volume 7, Polyurethane. 1st Edition, 1966, edited by Dr. R. Vieweg and Dr. A. Hxc3x6chtlen, 2nd Edition, 1983, edited by Dr. G. Oertel, and 3rd edition, 1993, edited by Prof. G. W. Becker and Prof. D. Braun, Carl-Hanser-Verlag, Munich, Vienna.
In comparison with the types of rubber which can be used in a similar manner, microcellular PU elastomers have significantly better damping properties with an excellent volume compressibility, so that they are used as constituents of vibration- and shock-damping systems, particularly in the automobile industry. To produce microcellular PU elastomers, useful starting materials have been found to be the reaction products of naphthylene 1,5-diisocyanate (1,5-NDI) and poly(ethylene glycol adipate) having a molecular weight of 2,000, which are reacted in the form of an isocyanate prepolymer with an activator-containing aqueous solution of a fatty acid sulfonate. (Kunststoff-Handbuch, Volume 7, Polyurethane, 1st Edition, page 270 ff.)
Since such base formulations give microcellular PU elastomers having very good damping characteristics and static and dynamic performance parameters, the prior art has disclosed only isolated efforts to replace the 1,5-NDI, which is responsible for the good elastomer properties but is more difficult to handle because of its high melting point, by more easily handled and less costly diisocyanates, since this results in significant losses in mechanical properties. Characteristic property differences between compact PU elastomers in general and microcellular PU elastomers in particular based on 1,5-NDI, tolylene diisocyanate (TDI) and 4,4xe2x80x2-MDI are shown in Journal of Elastomers and Plastics, Vol. 21 (1989), pages 100 to 121. Important disadvantages of a microcellular PU elastomer based on 4,4xe2x80x2-MDI are given as a distinctly higher degree of damping with increased heating of the material and significantly increased consolidation on dynamic loading, which finally lead to quicker material wear in comparison with PU elastomers based on 1,5-NDI.
Despite these obvious disadvantages, attempts have been made in the production of microcellular PU elastomers to replace the 1,5-NDI by the lower-melting and lower-cost 4,4xe2x80x2-MDI. However, these experiments have been restricted to the use of new starting components, in particular relatively high molecular weight polyhydroxyl compounds, the use of which has improved certain mechanical properties of the microcellular PU elastomers.
EP-A-0 496 204 (U.S. Pat. No. 5,173,518) describes a process for producing cellular PU elastomers using polyether polycarbonate diols, which comprise condensed polyoxytetramethylene glycol units having a mean molecular weight of from 150 to 500, as relatively high molecular weight polyhydroxyl compounds. This improves the mechanical properties, in particular the elongation at break, even at relatively low temperatures. However, it is not possible to recognize any improvement in the static compressive set values in accordance with DIN 53 572 at 70xc2x0 C. which are known to correlate with the dynamic consolidation values. Even when using 1,5-NDI as polyisocyanate, only average static compressive set values are obtained.
EP-B-0 243 832 (U.S. Pat. No. 4,798,851), which describes the use of pseudoprepolymers based on 4,4xe2x80x2-MDI, for example in combination with water as blowing agent, for producing elastic, compact or cellular PU or PU-polyurea moldings, teaches the use of a hydroxyl-containing polycondensate from a short-chain polyoxytetramethylene glycol and an aliphatic dicarboxylic acid as relatively high molecular weight polyhydroxyl compound, with the object of obtaining a polyhydroxyl compound containing ester groups and readily able to be metered by means of pumping for producing cellular or compact PU elastomers having improved mechanical and hydrolytic properties. Details of the permanent deformation on static or dynamic loading, by means of which vibration-resistant materials are usually characterized, are not disclosed.
DE-A-36 13 961 (U.S. Pat. No. 4,647,596) describes a microcellular PU elastomer based on 4,4xe2x80x2-MDI which, owing to a defined composition of the relatively high molecular weight polyhydroxyl compounds, comprising a copolymer of polytetrahydrofuran and xcex5-caprolactone, has mechanical properties which represent a useful compromise between static strength and dynamic stressability. Despite the use of expensive raw materials for producing the polyhydroxyl compounds, the gain in performance achieved thereby appears to be relatively small in terms of the test parameters xe2x80x9cproduct durability, flexural strength by the De Mattia method and permanent deformation at 50% compressionxe2x80x9d. For example, the measured values for the compressive set, which are directly related to the practically important value of the dynamic consolidation, show only slight improvements when the teachings of the invention are applied.
In addition, the test criteria used, viz. xe2x80x9cproduct durability and flexural strength by the De Mattia methodxe2x80x9d appear insufficiently suitable for an evaluation of the dynamic properties which is close to practice, since, especially in the case of partial property improvements, they are not able to give a satisfactory picture of the real performance differences between 4,4xe2x80x2-MDI-based and 1,5-NDI-based polyurethane elastomers. Thus, the example based on 1,5-NDI shows no better level of properties than the examples based on 4,4xe2x80x2-MDI.
Also known is the stepwise production of PU elastomers. According to DE-A-25 47 864 (U.S. Pat. No. 4,191,818), a heat-resistant PU elastomer can be produced by reacting an essentially linear relatively high molecular weight dihydroxyl compound with an excess of diisocyanate to give an adduct containing terminal hydroxyl groups and subsequently reacting this adduct with a symmetric aromatic diisocyanate in excess and alkanediols or di(alkylene glycol) terephthalates as chain extenders. If cellular PU elastomers are to be produced by this method, the chain extenders used can also be water, if desired in combination with alkanediols and/or di(alkylene glycol) terephthalates.
Cellular PU elastomers can also be produced by the process described in DE-A-2 940 856 (U.S. Pat. No. 4,334,033). According to this process, the relatively high molecular weight polyhydroxyl compounds and possibly chain extenders are reacted with an organic diisocyanate in a ratio of OH to NCO groups of from 1.2:1 to 2:1 to give a hydroxyl-containing prepolymer. This is divided in a weight ratio of about 80-20:20-80 into a component (I) and (II), the component (I) is reacted with 1,5-NDI in a ratio of OH:NCO groups of 1:2.5-12 to give an NDI-polyurethane adduct containing NCO groups and the component (II) is combined with chain extenders, water and additives to give a mixture (II). The NDI-polyurethane adduct and the mixture (II) are finally reacted to give a cellular or non-cellular PU elastomer. According to this process, the formative components can be metered exactly and mixed rapidly and intensively. The PU elastomers are homogeneous and have uniform mechanical properties over the entire molding.
Curable PU-polyurea prepolymer compositions based on prepolymers containing isocyanate groups and prepared from hydroxyl-containing poly(ethanediol adipates), 2,4-TDI and TODI and hardeners comprising 2-chloroaniline-2,5-dichloroaniline-formaldehyde condensation products are described in GB-A 1 141 323. The compact PU-polyurea elastomers produced therefrom have good mechanical properties, however, the carcinogenic action of the hardeners used is a disadvantage.
It is an object of the present invention to provide a process for producing compact or preferably microcellular PU elastomers in which the expensive TODI can be replaced completely or at least partially by organic diisocyanates which are easier to handle and have a lower cost. Despite the concomitant use of other organic diisocyanates, the mechanical and dynamic properties of the PU elastomers produced should be improved or at least correspond essentially to elastomers based on TODI. Regardless of the type of relatively high molecular weight polyhydroxyl compounds used, the microcellular PU elastomers should have static and mechanical properties which are clearly improved in comparison with PU elastomers based on 4,4xe2x80x2-MDI, in particular they should have compressive sets and dynamic consolidation values such that they can be used, in particular, for producing vibration- and shock-damping systems.
We have found that this object is achieved by a process for producing compact or cellular, preferably microcellular, PU elastomers by reacting
a) relatively high molecular weight polyhydroxyl compounds and, if desired,
b) low molecular weight hydroxyl-containing chain extenders and/or crosslinkers with
c) organic polyisocyanates
in the presence or absence of
d) catalysts,
e) blowing agents and
f) additives,
wherein the organic polyisocyanates used are 3,3xe2x80x2-dimethylbiphenyl 4,4xe2x80x2-diisocyanate and at least one additional aromatic diisocyanate selected from the group consisting of tolylene diisocyanate, diphenylmethane diisocyanate, 1,2-diphenylethane diisocyanate and phenylene diisocyanate, and/or at least one aliphatic diisocyanate having from 4 to 12 carbon atoms and/or at least one cycloaliphatic diisocyanate having from 6 to 18 carbon atoms.
According to a preferred embodiment, the PU elastomers are produced by the prepolymer process in which a polyaddition product containing urethane and isocyanate groups is advantageously prepared from the relatively high molecular weight polyhydroxyl compound (b) and at least one aromatic diisocyanate selected from the group consisting of TDI, MDI, 1,2-diphenylethane diisocyanate (DIBDI), phenylene diisocyanate (PDI) and preferably 4,4xe2x80x2-MDI and/or hexamethylene 1,6-diisocyanate (HDI) and/or 1-isocyana-to-3,3,5-trimethyl-5-isocyanatomethylcyclohexane (IPDI), and this polyaddition product is converted into the prepolymer containing isocyanate groups by reaction with TODI which can be reacted with the polyaddition product in one step or in portions. Microcellular PU elastomers can be produced from such prepolymers containing isocyanate groups by reaction with water or mixtures of water and, if desired, low molecular weight hydroxyl-containing chain extenders and/or hydroxyl-containing crosslinkers (b) and/or relatively high molecular weight polyhydroxyl compounds (a).
The invention also provides prepolymers containing isocyanate groups and having an NCO content of from 3.3 to 10% by weight, preferably from 3.5 to 9.0% by weight, which are prepared by reacting at least one relatively high molecular weight polyhydroxyl compound (a) or a mixture of (a) and at least one low molecular weight hydroxyl-containing chain extender and/or cross-linker (b) with at least one aromatic diisocyanate selected from the group consisting of MDI, DIBDI and PDI, preferably 4,4xe2x80x2-MDI, and/or HDI and/or IPDI to give a polyaddition product containing urethane and isocyanate groups and having an NCO content of advantageously from 0.05 to 8% by weight, preferably from 1.2 to 7.5% by weight, and reacting this polyaddition product with TODI which can be incorporated into the reaction mixture and reacted with the polyaddition product in one step or preferably in portions.
Since the ability to crystallize of the rigid segments comprising urea and/or urethane groups in the PU elastomers based on 1,5-NDI or TODI is considerably impaired by the concomitant use of diisocyanates which crystallize less well, for example the low-cost 4,4xe2x80x2-MDI with its angled structure, those skilled in the art had to assume that the resulting PU elastomers have poorer static and dynamic properties than those based on an aromatic diisocyanate.
It could therefore not be foreseen that the microcellular PU elastomers produced from specifically selected aromatic, aliphatic and/or cycloaliphatic diisocyanate mixtures containing TODI have good mechanical properties which are almost comparable with those of elastomers produced exclusively from TODI and, in comparison with microcellular PU elastomers based on 4,4xe2x80x2-MDI, have clearly improved static, mechanical properties, in particular compressive set and dynamic consolidation values. The microcellular PU elastomers produced by the process of the present invention are thus lower in cost than PU elastomers based on 1,5-NDI or TODI and, owing to their good static, mechanical properties, are very useful for producing vibration- and shock-damping systems. Furthermore, the reaction mixture is simpler to handle and process.